A communication system includes a first communication device configured to transmit input data to a transmission path, detects a first accumulated amount of the input data, and transmit notifying data to the transmission path when the detected first accumulated amount has reached a threshold value, and a second communication device configured to receive data transmitted to the transmission path by the first communication device, detect a second accumulated amount of the received data, and output measurement information based on the detected second accumulated amount and the threshold value when having received the notifying data.